This invention relates to a stage system and, more particularly, a stage control system wherein, in addition to feedback of a position signal, feedback of a velocity signal is performed to thereby improve the position settling characteristic. The stage control system can be suitably used in a semiconductor exposure apparatus, for example, to increase the precision and productivity.
The requirement applied to a kinetic mechanism is simple. It is to move a thing fast and precisely. In order to meet this, the basic stance of the mechanical design should pursue a kinetic mechanism of light weight and high rigidity. To this end, mechanical designs have been made with the use of a material or materials having a high specific rigidity. However, when motion of a mechanism is considered, namely, when the mass (M), rigidity (K) and viscous damping (C) are taken into account, it can be stated that only M and K are designed in conventional mechanical designs. Practically, when a light weight and high rigidity kinetic mechanism is manufactured and then control is made thereto, the problem which arises therefrom is a high frequency resonance phenomenon. When a light weight and high rigidity structure is accomplished, a mechanical vibration in a high frequency region can be produced, as is well known in the art. Here, when the control is conventional, that is, if the setting is such that the kinetic mechanism does not move fast, the mechanical resonance will be in a high region outside the control band and, therefore, it will not adversely affect the kinetic performance. However, the controller for a light weight and high rigidity kinetic mechanism is so adjusted to move the same fast. Namely, the closed loop frequency characteristic is set in a higher frequency region. As a result, even when a light weight and high rigidity kinetic mechanism is manufactured and the resonance frequency of the mechanism is set to be a high frequency, since the closed loop frequency response is also at a high frequency, the resonance of the mechanism largely influences the kinetic performance.
This problem is attributable to that, in the mechanical design, the viscous damping (C) is not designed in addition to the mass (M) and rigidity (K). This means that the viscous damping (C) should be designed in the mechanical design. Conventionally, however, there is no established design formula in regard to the viscous damping (C) and, therefore, it is not possible to fully meet the requirement. What can be best done may be predicting a possible mechanical resonance in the mechanism designing and conveniently inserting a viscous damping element such as a rubber element. Alternatively, if an unexpected resonance occurs after the mechanism is manufactured, similarly a viscous damping element such as a rubber element may be inserted to apply a damping function to suppress the vibration. As described above, in order to provide a fast and high precision kinetic mechanism, how to design the damping function is very important.
In the situations described above, in the field of kinetic control, research and development on the xe2x80x9cdamping technologyxe2x80x9d is a very important theme. In fact, there is a special committee in the Japan Society of Mechanical Engineers in regard to the damping technology. The importance and extension of this field can be recognized.
In the field of motion control of a mechanism, because of difficulties in applying a damping function by use of mechanism means, generally the damping control is made by the feedback of an output of an acceleration sensor. For example, use of an acceleration sensor is inevitable, in a suspension control of vehicles for better comfort or an active vibration control unit (usually called an xe2x80x9cactive mass damperxe2x80x9d) for vibration control of a structure. This is because high sensitivity and small size acceleration sensors are easily obtainable and they can be incorporated into a controlled object. However, from the viewpoint of damping application to a kinetic mechanism, an output of a velocity sensor rather than an acceleration sensor directly represents a physical amount.
Now, the damping application will be considered with respect to an X-Y stage in a semiconductor exposure apparatus, as a representative example of a high precision kinetic mechanism. To such an X-Y stage, there is a feedback system based on an output of a laser interferometer. For example, in a position control system, an output signal of a laser interferometer is compared with a position command profile, and a positional deviation signal is produced. Then, PID compensation is made to this deviation signal to energize an actuator for driving the X-Y stage. A closed loop is thus accomplished. The control is made only to a single loop based on the output of the laser interferometer. Here, P denotes xe2x80x9cproportionalxe2x80x9d, I denotes xe2x80x9cintegrationxe2x80x9d, and D denotes xe2x80x9cdifferentiationxe2x80x9d. What applies damping to the motion of the stage is only D (differentiation) in the PID compensation.
With such a single feedback loop, however, practically, it is difficult to perform the adjustment fully satisfying the requirements applied to the X-Y stage. If possible, the performance to follow a position command input and the performance of suppression to an external disturbance input must be shared. If a velocity feedback loop can be added, in addition to the position feedback loop, it effectively contributes to shortening the settling time. From the point of design control, it is evident that the control performance can be improved by multiplying the feedback loop. Practically, however, the control loop of the stage is not multiplied. This is because there are physical restrictions in machine design or the effectiveness of multiplying the feedback loop is not known. Alternatively, there is no specific way for multiplying the control system of an X-Y stage. Namely, there is not exact knowledge as to which physical amount of the X-Y stage should be measured to add a loop inside the feedback loop based on the output of the laser interferometer.
The present invention aims at improving the position settlement, in the positioning control of a precision positioning mechanism such as an X-Y stage of a semiconductor exposure apparatus, for example, by using a damping loop in addition to a position feedback loop based on a laser interferometer, for example.
Attempts for improving the positioning settlement performance of an X-Y stage through the damping application are made in some documents. For example, Japanese Laid-Open Patent Application, Laid-Open No. 237061/1995, shows a stage system wherein a protrusion is formed on the bottom face of a movable stage and wherein the protrusion is dipped in a viscous fluid. By moving the protrusion upwardly and downwardly, the contact amount with the viscous fluid changes to thereby adjust the viscous resistance. Thus, in this stage system, by adjusting the damping amount, the movable stage can be positioned at a predetermined position without bunching. In the stage system disclosed in this document, the damping is applied by arranging the mechanism specifically. Also, Japanese Laid-Open Patent Application, Laid-Open No. 170990/1996 shows a stage system wherein, as a viscous fluid, an ER (Electro-Reological) fluid having a viscous resistance coefficient variable with an electric field is used, and wherein the damping coefficient based on the viscous resistance coefficient of the viscous fluid is changed by using a control system to thereby provide a variable rigidity of an anti-vibration mount. In this manner, occurrence of vibration or the like due to the movement of the stage is suppressed.
In the former document, the viscous damping element is incorporated into the stage itself as its structure. It is seen from this document that many efforts have been made to provide the damping application to shorten the positioning settlement time of the stage. In this document, the damping is applied mechanically and, therefore, it needs complicated maintenance and adjustment operations. From the viewpoint of easy maintenance and for better absorption of dispersion of the stage positioning characteristic, applying the damping through the feedback control is preferable. The present invention provides a stage control system wherein the damping is applied through a feedback control.
As described above, a semiconductor exposure apparatus includes a stage for precise positioning. In order to meet the high speed drive, it uses a static pressure for the guide. In such a non-contact guide, in most cases, there is substantially no damping element. Therefore, when the stage is driven, it can be moved faster because of no resistance. However, when the stage is stopped, the settling characteristic is not good because there is no damping element. In conventional stage control systems, an output of a laser interferometer for measuring the position of the stage is fed back, and a differentiating element is added to the compensation calculation of this loop by which the damping is applied to the stage. However, the damping applied to such a feedback loop is insufficient.
Further, the stage mechanism itself involves high frequency resonance. Usually, such mechanical resonance cannot be removed unless, during the mechanism design, some positive measurements of selecting a special material to produce internal attenuation are taken. With increased velocity of the stage, mechanical resonance is easily excited. Unless the damping is applied thereto, it is difficult to accomplish high speed drive of the stage as well as stopping it in a short time and with a good precision.
It is an object of the present invention to solve at least one of the problems described above.
In accordance with an aspect of the present invention, there is provided a stage system, comprising: a movable stage; position measuring means for measuring the position of said stage; a velocity sensor for detecting the velocity of said stage; and a control unit having a position feedback loop based on an output of said position measuring means and a feedback loop for applying a damping to said stage on the basis of an output of said velocity sensor.
Preferably, the stage system may further comprise (i) a position compensator for performing predetermined compensation to a position error signal determined on the basis of an output of said position measuring means and a command position, (ii) a velocity compensator communicated with said position compensator to direct an output of said velocity sensor as a negative feedback signal, and (iii) an electric current amplifier for energizing an actuator for driving said stage in accordance with an output of said velocity compensator.
Preferably, the stage system may further comprise (iv) a base plate for movably supporting said stage, and (v) an acceleration sensor for detecting an acceleration of said base plate, wherein said control unit includes means for applying a signal, corresponding to an output of said acceleration sensor as multiplied by a predetermined gain, to said current amplifier as positive feedback.
The stage may be supported by a base plate without contact thereto, with use of a static pressure bearing.
The stage may be driven by a linear motor.
In accordance with another aspect of the present invention, there is provided an exposure apparatus, comprising: a movable stage; position measuring means for measuring the position of said stage; a velocity sensor for detecting the velocity of said stage; and a control unit having a position feedback loop based on an output of said position measuring means and a feedback loop for applying a damping to said stage on the basis of an output of said velocity sensor.
Preferably, the apparatus may further comprise (i) a position compensator for performing predetermined compensation to a position error signal determined on the basis of an output of said position measuring means and a command position, (ii) a velocity compensator communicated with said position compensator to direct an output of said velocity sensor as a negative feedback signal, and (iii) an electric current amplifier for energizing an actuator for driving said stage in accordance with an output of said velocity compensator.
Preferably, the apparatus may further comprise (iv) a base plate for movably supporting said stage, and (v) an acceleration sensor for detecting an acceleration of said base plate, wherein said control unit includes means for applying a signal, corresponding to an output of said acceleration sensor as multiplied by a predetermined gain, to said current amplifier as positive feedback.
The stage may be supported by a base plate without contact thereto, with use of a static pressure bearing.
The stage may be driven by a linear motor.
In these aspects of the present invention, a velocity sensor is provided and an output thereof is fed back so as to apply a damping to the stage motion. Thus, there is a velocity feedback loop for applying the damping, in addition to the position feedback loop. This enables a strong damping effect by adjustment of the damping only.